Gizmo types in blender 2.8
One of the great thing about blender is that when something is not documented you can just look at the source. :)

Here are the ones I've found (untested):

GIZMO_GT_arrow_2d

GIZMO_GT_arrow_3d


props:


offset

draw_style:


NORMAL | CROSS | BOX| CONE

transform:


INVERT| CONSTRAIN



GIZMO_GT_blank_3d

GIZMO_GT_button_2d


props:


icon | shape| show_drag

draw_style:


OUTLINE | BACKDROP| HELPLINE



GIZMO_GT_cage_2d


props:


dimensions

draw_style:


BOX | CIRCLE



GIZMO_GT_cage_3d

GIZMO_GT_dial_3d

GIZMO_GT_move_3d

GIZMO_GT_primitive_3d

GIZMO_GT_value_2d


props:


offset



VIEW3D_GT_navigate_rotate

GIZMO_GT_mesh_preselect_elem_3d


props:


object_index | vert_index| edge_index | face_index



GIZMO_GT_mesh_preselect_edgering_3d

VIEW3D_GT_ruler_item

I'm actually working on custom gizmos so I'll update this answer to document my findings.